Structural states of myelin observed by x-ray diffraction and freeze-fracture electron microscopy.

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Abstract
Coordinated freeze-fracture EM and X-ray diffraction were used to visualize the morphological relation between compacted and native period membrane arrays in [mouse] myelinated nerves treated with dimethylsulfoxide (DMSO). Comparison of X-ray diffraction at room temperature and at low temperature was used as a critical measure of the extent of structural preservation. The X-ray diffraction patterns show that in the presence of cryoprotective agents, it is possible to preserve with only small changes the myelin structure which exists at room temperature. The changes include a slight increase in packing disorder of the membranes, a small, negative thermal expansion of the membrane unit and some reorganization in the cytoplasmic half of the bilayer. The freeze-fracture EM clearly demonstrates continuity of compact and native period phases in DMSO-treated myelin. The use of freezing to trap the transient, intermediate sturcture during a structural transition in glycerol was demonstrated.